The present invention relates to novel, strictly linear polyimides and a process for their production.
Polyimides are usually obtained by reacting either
(a) tetracarboxylic acid dianhydrides with diamines, PA0 (b) tetracarboxylic acid dianhydrides with diisocyanates, and PA0 (c) difunctional cyclic dienes with N,N'-bis-maleic acid imides. PA0 p-phenylene-bis-maleic acid imide PA0 m-phenylene-bis-maleic acid imide PA0 4,4'-bis-maleic acid imidodiphenylmethane PA0 4,4'-bis-maleic acid imidodiphenyl ether PA0 4,4'-bis-maleic acid imidodiphenylsulfone PA0 4,4'-bis-maleic acid imidobenzophenone PA0 1,2-bis-maleic acid imidobutane PA0 1,6-bis-maleic acid imido-2,2,4- and/or -2,4,4-trimethylhexane PA0 1,6-bis-maleic acid imidohexane PA0 1,12-bis-maleic acid imidododecane PA0 2-methyl-1,5-bis-maleic acid imidopentane PA0 2-ethyl-1,4-bis-maleic acid imidobutane. PA0 benzo(1,2:4,5)dicyclobutene PA0 3,6-dimethylbenzo(1,2:4,5)dicyclobutene PA0 3,6-dibromobenzo(1,2;4,5)dicyclobutene.
The reaction of tetracarboxylic acid dianhydrides with diamines (cf. U.S. Pat. Nos. 3,677,921; 4,094,862; 4,282,461; 4,063,345 and German Pat. Nos. 1,946,925 and 1,947,029) is a two-stage process. In the first stage, polyamide carboxylic acids are produced which, in a second stage, are reacted at high temperatures to form polyimides. One disadvantage encountered in this process is the occurrence of crosslinking in the second stage imide formation, and the resulting cross-linked polyimides can not be processed by thermoplastic methods.
Ways to solve this problem are set forth in U.S. Pat. Nos. 3,905,442; 3,983,093; 3,968,083; 3,991,004; German Pat. Nos. 2,261,715; 2,202,102; 2,363,785; 2,416,595; 2,441,101; and DOS No. 3,200,633; however, these processes, though avoiding this drawback, require special monomers and/or catalysts that can only be produced at great cost.
In the reaction of tetracarboxylic acid dianhydrides with diisocyanates, as disclosed in U.S. Pat. Nos. 3,787,367 and 3,870,677 and in J. Polym. Sci. Polym. Chem. Ed., 18: 1905-1909 (1980), non-crosslinked and processable polyimides are obtained in one stage, but one disadvantage of the process is the use of diisocyanates which must usually be prepared from the diamines utilized in the first process above, involving an additional reaction step requiring the use of toxic phosgene.
Linear polyimides may also be obtained in a reaction of N,N'-bis-maleic acid imides with bis-dienes [cf. U.S. Pat. No. 3,344,071, Macromol. Symp. 28: 611 (1982), Eur. Polym. J. 13: 179 (1977), J. Polym. Sci. Polym. Chem. Ed. 11: 2143 (1973), and Vysokomol. Soedin, Ser. A 18: 926 (1976) all of which disclosures are incorporated by reference herewith], or with dienes with cheletropic groups (German Pat. Nos. 1,100,291; 1,124,690; and 1,145,797 which disclosures are incorporated by reference herewith). A fundamental drawback in this reaction is the fact that such "poly-Diels-Alder reactions" are reversible at an elevated temperature.
Consequently, the polyimides produced according to the state of the art are either crosslinked, thermally unstable or manufacturable only at great cost.